2. BACKGROUND AND MOTIVATION FOR ULTRAVIOLET IMAGING OF
GALAXIES

Changes in the two-dimensional light distribution of galaxies with
wavelength can provide new and unique perspectives on their structures
and evolution. The spectral region of interest here lies between the
Lyman cutoff of the interstellar medium at 912 Å and the cutoff of
the terrestrial atmosphere near 3200 Å. This window contains
information on the character of stellar populations, dust grains,
interstellar gas, and AGNs which is largely independent of that in the
familiar optical bands. In this section we discuss the potential utility
of the UV in understanding galaxy evolution and progress to date in
exploiting these opportunities.

The UV has the highest sensitivity of any spectral region to stellar
temperature and metal abundance, implying that it is especially valuable
as a means of characterizing stellar populations, current star formation
rates (SFRs), and star formation histories. Stars with surface
temperatures above ~ 10,000 K (e.g., main-sequence stars with masses
3
M) are
brighter in the UV than at longer wavelengths
(Fanelli et al. 1992).
Consequently, UV imaging or spectroscopy of star-forming galaxies
permits direct detection of the massive stars responsible for most
ionization, photodissociation, kinetic energy input, and element
synthesis.

Figure 1 illustrates predicted UV-IR spectral energy
distributions of stellar populations over timescales up to 3000 Myr. The
strong time evolution of the UV compared to longer wavelengths is the
reason for its utility in determining population ages. The sensitivity
of the UV to stellar properties extends even to the cool ~ 1
M stars near
the main-sequence turnoff in the oldest model shown in
Figure 1. These solar-type stars dominate the mid-UV
(2000-3200 Å) light in this model, and chemical composition as well
as age influences the spectrum. The concentration of strong metallic
absorption features is responsible for much of the short-wavelength
structure in this model. The abundance sensitivity of selected UV
spectral regions is discussed by
Fanelli et al. (1992).

Figure 1. Synthetic spectral energy
distributions of single generation stellar populations having Salpeter
IMFs and solar abundances for ages 3-3000 Myr from
Bruzual & Charlot (1993),
showing the rapid evolution in UV amplitude and shape. The shape of the
near-IR spectrum ( >
7000) is much less sensitive to age. Interstellar emission lines are
not modeled here but would be absent in all except the 3 Myr SED.

In old, quiescent systems such as elliptical galaxies and spiral bulges,
the UV offers a second major probe of stellar populations. Most old
systems have been found to contain a very hot, low-mass stellar
component with Teff > 15,000 K which dominates the
far-UV light. This probably consists of stars with very thin envelopes
on the extreme horizontal branch and subsequent advanced evolutionary
phases (reviewed in
Greggio & Renzini 1990
and
O'Connell 1999).
Their UV output is predicted to be very sensitive to their envelope
masses and compositions. Overall, UV spectra are powerful age and
abundance diagnostics for both young and old populations.

The UV offers a unique probe of the star formation history of galaxies
on intermediate timescales of 10-1000 Myr. By "history" we mean the star
formation rate as a function of time, SFR(t). The most widely
used methods for estimating the recent star formation rate,
SFR(t0), involve optical emission lines such as
H, radio continuum emission
from hot gas or relativistic electrons, and far-infrared or
submillimeter continuum emission from dust grains (e.g.,
Kennicutt 1998).
Both emission lines and free-free thermal radio continuum depend on
photoionization from massive stars and therefore reflect activity only
over the past ~ 5 Myr, after which photoionization rapidly
decreases. This period represents only 0.05% of the star-forming
lifetime of a typical galaxy. Nonthermal radio emission powered by
supernova-driven relativistic electrons is a useful index of massive
star formation over the past few tens of Myr
(Condon 1992)
but is influenced by the character of the surrounding interstellar
medium. Infrared ( > 10
µm) and submillimeter thermal emission from dust grains is
likewise strongly influenced by the nonstellar environment and has
intrinsically poor time resolution, since grains can be heated by
photons from stars of all ages.

These conventional methods for estimating recent SFRs
are based on the indirect effects of massive stars, involving the
downconversion of UV photons by surrounding media, and have either
restricted or ill-defined age sensitivity. By contrast, the vacuum UV
offers a direct measure of the light from the massive star
populations. Extinction by dust is often cited as a serious obstacle to
using direct FUV observations to infer star formation rates. However,
the photoionizing UV continuum
( 912 Å) which
drives line and free-free emission is possibly yet more sensitive to
extinction, while there are fewer empirical constraints on its
nature. The actual effects of dust on the emergent UV light are smaller
than naively expected (see Section 2.1.3 and
6). All of
these methods are comparably affected by uncertainties in extinction.

The timescales which can be probed by observations of
the 1200-3200 Å continuum range from less than 10 Myr to
1 Gyr. This critical
interval is well sampled neither by the methods described above nor by
the optical region (3200-9000 Å), which is influenced by the star
formation history on longer timescales (more than a few Gyr). It is the
"gap" which
Gallagher, Hunter, & Tutukov (1984),
for instance, were compelled to omit in their landmark study of spiral
galaxy histories derived from
H fluxes, B-band
fluxes, and total masses.

An example of the additional information on galaxy star formation
histories to be gained from UV continuum imaging is shown in
Figure 2. This is a map comparing the
H and far-UV morphologies of
the well-known Sc galaxy M51. There are clearly large variations in the
far-UV to H ratio. The ~
10-50 Myr old populations (FUV-bright) are usually spread farther
downstream from the putative density wave than the ~ 5 Myr old,
H -bright populations, but
the pattern is not entirely symmetrical. Diffuse far-UV light tends to
"fill in" the spiral arms between intense concentrations of
H light. There is a hint of
multiple FUV wavelets, with feathery extensions inclined in pitch angle
to the main spiral pattern. The UIT data for M51 are discussed further
by
Kuchinski et
al. (2000).
A similar comparison map based on a lower resolution UV image from the
FOCA program (see Section 2.3) was published by
Petit et al. (1996).

Figure 2.
H-FUV difference map of the
face-on Sc galaxy M51. The image shows the logarithmic difference
between a continuum-subtracted ground-based
H image and a UIT far-UV
(1500 Å) continuum image taken by UIT during the Astro-2
mission. The map contrasts regions of current star formation
(5 Myr), which are
bright in H and appear as
white/light gray in the image, with regions active during the era 5-50
Myr in the past, which are relatively brighter in the FUV continuum and
are represented by dark gray/black in the image.

The sensitivity of different wavebands to a galaxy's star formation
history is discussed further in the form of "history weighting
functions" in
Appendix A.

The UV offers high sensitivity to interstellar dust and regions of
concentrated cold material (i.e., potential star formation
sites). Before the advent of UV observations of galaxies, it was widely
assumed that this would actually be a serious disadvantage because the
Galactic extinction law (e.g.,
Cardelli, Clayton, & Mathis 1989)
yields A(UV) / A(V) > 2.5, implying that the UV
light of typical disk galaxies might be strongly suppressed. However, as
is amply demonstrated by the images in this atlas and spectroscopic
studies (e.g.,
Calzetti, Kinney, & Storchi-Bergmann
1994),
dust does not dominate the UV morphology of most galaxies.

The UV may ultimately prove to be a valuable tracer of
quiescent, cold, molecular material. Because the albedo of dust is high
in the UV, cool interstellar clouds far from star-forming regions can be
detected by scattered light, as in the case of the faint gaseous outer
arms of M101
(Donas et al. 1981;
Stecher et al. 1982)
or the outer halo of NGC 1068
(Neff et al. 1994).
UV imaging can also detect H2 in photodissociation regions
directly by virtue of its fluorescence bands in the 1550-1650 Å
region (e.g.,
Witt et al. 1989;
Martin, Hurwitz, & Bowyer 1990).
Although such regions occupy only a small fraction of the total volume
of a typical molecular cloud, nonetheless the direct detection of
H2 has, in principle, considerable advantages over methods
involving trace constituents like radio-emitting CO (see
Allen et al. 1997
and references therein).

The UV contains uniquely important emission line probes of interstellar
gas in the T ~ 105-106 K regime, including
C IV (1550),
N V (1241), and
O VI (1035). These
spectral diagnostics have been extensively exploited in absorption-line
spectroscopic studies of our galaxy. Little has been done to date using
imaging, though C IV images of supernova remnants have been published
(e.g., the Cygnus Loop,
Cornett et al. 1992;
N49 in the LMC,
Hill et al. 1995c).

UV imaging of nearby galaxies is relevant to galaxies at high redshift
for two reasons. First, as just described, the rest-frame UV continuum
is a robust tracer of star formation and is measurable to very high
redshifts (z
10) with optical/IR instruments. For instance, the 2800 Å
rest-frame continuum has been used to estimate the cosmic star formation
density at z ~ 0.5-4 for the Canada-France Redshift Survey,
Hubble Deep Field, and other surveys
(Pei & Fall 1995;
Lilly et al. 1995;
Madau et al. 1996;
Steidel et al. 1999),
leading to the
conclusion that gas processing occurred at a relatively constant rate
for z ~ 1-4 but has precipitously declined since z ~ 1.

Second, observations of high-z galaxies are
preferentially made in the rest-frame UV. This is particularly true for
ground-based telescopes, where the rapidly increasing night sky
brightness for wavelengths above 7000 Å, and thermal emission
beyond 2 µm, seriously compromises observations in the near
infrared. Because of the strong changes in galaxy appearance with
wavelength, as illustrated in this atlas, there is a large
"morphological k-correction" which must be quantified in order to
distinguish genuine evolutionary effects from simple bandshifting.

High-redshift studies are also strongly influenced by reduced spatial
resolution and by surface brightness selection. The latter is a very
serious problem for z 1 because I ~
I0(1 + z)-n, where
I0 is the surface brightness in the rest-frame and
n = 3 or 5 for monochromatic surface brightnesses per unit
frequency or wavelength, respectively; n = 4 for bolometric
surface brightnesses.
Figure 3 illustrates these effects using a
far-UV image of the luminous, nearby Sc spiral M101.
Bohlin et al. (1991)
and Giavalisco et
al. (1996)
describe methods of creating such simulations from rest-frame UV data.

Figure 3.Left panel: A far-UV (1500
Å) image of the luminous Sc galaxy M101 obtained by UIT during the
Astro-2 mission. A 5' bar is shown for scale. Right panel: A
simulation of a galaxy with the same structure but 10 times higher
surface brightness at a redshift z = 1.5 as observed by the Keck
10 m telescope in a 10 hr exposure with 0".5 FWHM seeing against a
sky background of 22.5 mag arcsec-2. A 2" bar is shown
for scale. The simulation is not easily recognizable as a normal spiral
galaxy. Its asymmetries are emphasized; it appears distorted and
fragmented. High surface brightness star-forming associations in its
disk have taken on the appearance of nearby "companions."

It is possible to explore bandshifting effects using multicolor (e.g.,
B and R) optical images to extrapolate the spectral energy
distribution to the rest-frame UV on a pixel-by-pixel basis. This has
been done using ground-based data (e.g.,
Abraham 1997;
Abraham, Freedman, &
Madore 1997;
Brinchmann et al. 1998)
and moderate-redshift Hubble Space Telescope (HST) data
(Bouwens, Broadhurst,
& Silk 1998).
These studies, as well as those based on HST/NICMOS observations
in the rest-frame optical bands (e.g.,
Bunker 1999,
Corbin et al. 2000),
conclude that the peculiarities in shape and size distributions found in
the deep HST surveys considerably exceed the effects of
bandshifting. While this is probably a robust result, these
extrapolation methods do not accurately capture the range of rest-frame
UV spectra found in real galaxies and are not suitable for making
detailed comparisons with the local universe. The reason is that there
is considerable scatter in UV colors of nearby galaxies at any given
optical color. This is true even in the classic (U-B,
B-V) diagram (e.g.,
Larson & Tinsley
1978),
but it is much more pronounced in the rest-frame UV, where, for
instance,
Donas, Milliard, &
Laget (1995)
found a 3 mag range in (UV-b) colors at a given
(b-r) color in a faint galaxy sample. This UV/optical
decoupling is confirmed in spectroscopy of UV-selected samples by
Martin et
al. (1997) and
Treyer et al. (1998).
The implication is that the true evolutionary history of galaxies on
timescales more recent than a few Gyr can be rather different from that
inferred from optical data.

Fiducial photometric and imaging studies of nearby galaxies in the
rest-frame UV are needed in order to calibrate these selection and
morphological effects and to improve our understanding of the
astrophysical drivers of the rest-frame UV luminosity, particularly the
influence of dust and the history of star formation.

Extragalactic UV astronomy to date has been largely based on
spectroscopy, usually with small entrance apertures (1"-20")
centered on galaxy nuclei (e.g., IUE, HST/FOS,
HST/GHRS). Several hundred objects have also been photometered in
broad bands with large apertures. Early photometric surveys were
performed by OAO and ANS; the most recent large-scale study was by FAUST
(Deharveng et al. 1994).
The total number of UV spectroscopic and photometric observations of
galaxies still far exceeds the number of imaging observations (an
inversion of the historical development of optical extragalactic
astronomy).
Brosch (1999)
has recently reviewed the available results of UV surveys of
galaxies. No all-sky UV survey faint enough to include galaxies has yet
been conducted, but this will be remedied by the GALEX mission
(Martin et al. 1999).

The first UV image (defined as having many resolution elements
over the area of interest) of another galaxy was obtained by the NRL
Apollo S201 camera from the lunar surface in 1972
(Page & Carruthers
1981).
This was of the Large Magellanic Cloud in the 1250-1600 Å band and
dramatically demonstrated a strong wavelength-dependent morphology. Its
remarkably fragmented appearance in the UV is entirely different from
its familiar barlike shape in the optical continuum.

Subsequent progress in UV imaging up to 1990 was relatively slow
(reviewed in
O'Connell 1991).
Since 1990, we have accumulated a sample of vacuum UV images of about
200 galaxies, principally from three instruments:

The Hubble Space Telescope Faint Object Camera
(HST/FOC). Because of its better rejection of red leaks (i.e.,
residual response to low-energy photons outside the primary UV bands)
the HST/FOC has been more extensively used for UV imaging than
the HST/Wide Field Planetary Cameras. Its main limitation is a
small field of view. FOC produced the first extensive compilation of
galaxy UV images in an atlas of the nuclei (22" fields) of 110
nearby galaxies at 2300 Å with 0".05 resolution
(Maoz et al. 1996).
Many of the objects exhibit UV-bright nuclear structures, including a
number of star-forming rings and five cases of unusually bright LINER
active nuclei
(Maoz et al. 1995).
Far-UV (<2000 Å) imaging even with FOC is compromised by red
leaks, but the newer HST/Space Telescope Imaging Spectrograph
camera provides pure UV imaging over 25" fields (e.g.,
Brown et al. 2000).

The SCAP/FOCA balloon-borne telescope of the Marseille and Geneva
groups. SCAP/FOCA operates in a narrow atmospheric window near 2000
Å with a large field of view of up to 2°.3
and resolution of ~ 15"
(Deharveng et al. 1980;
Milliard, Donas, &
Laget 1991;
Donas et al. 1995).
By virtue of a
fast f/ratio and long integration times, it can reach relatively
low UV surface brightnesses despite its narrow band. A number of bright
galaxies have been studied morphologically and photometrically with
SCAP/FOCA.
Donas et al. (1987)
summarized integrated UV photometry for a sample of 149 spiral and
irregular galaxies. A sample of fainter, mostly unresolved, UV-detected
galaxies near the north galactic pole has been recently discussed by
Donas et al. (1995) and
Treyer et al. (1998).
Fewer objects have been studied with resolved imaging;
individual citations are given in Section 5.

The Ultraviolet Imaging Telescope (UIT), from which the UV images in
this atlas were obtained. UIT has a field of view of 40' and spatial
resolution of 3" and was intended to provide a good match to the
performance of typical ground-based telescopes on large angular
diameter, nearby galaxies. UIT is described in detail in the next
section.

Because of technical difficulties in achieving high reflectivity optics
shortward of 1100 Å and in rejecting the very bright geocoronal
Ly line at 1216 Å from
exposures centered at shorter wavelengths, both the HST and UIT
imaging cameras work primarily at wavelengths longer than 1216 Å.